In this study, we will examine questions regarding both the afferents to and the efferent actions of one group of basal ganglia output neurons: those of the entopeduncular nucleus, the feline internal globus pallidus. On the afferent side, we will electrophysiologically investigate the synaptic actions on ENTO cells of several brainstem sites identified in our recent HRP studies as potential sources of direct afferents to the internal globus pallidus. This may help to clarify what drives the high tonic firing rate characteristic of pallidal basal ganglia output neurons in awake animals. On the efferent side, we will use combined neuroanatomical and electrophysiological techniques to work out the patterns of convergence of information from ENTO and other sources on thalamocortical neurons. First, we will make physiologically-controlled localized injections of HRP and radioisotopes into the ENTO projection area of the thalamus and use the results to define the cortical destination of these thalamic neurons and the sources of afferent information to this part of the thalamus. Then we will stimulate the anatomically-defined afferent sources and cortical projection sites and record the patterns and combinations of synaptic action from ENTO and other sources on individual, antidromically-identified thalamocortical neurons. The total results will greatly extend out understanding of how basal ganglia output is controlled and interacts with information from other neural sources.